1. Field of the Invention
The present invention relates to a current sensor using a magneto-resistance effect element (TMR element or GMR element).
2. Description of the Related Art
In an electric vehicle, a motor is driven by electricity generated by an engine, and the intensity of the current for driving the motor is detected by, for example, a current sensor. The current sensor includes a magnetic core placed around a conductor and having a cutaway portion (core gap) formed at a portion thereof, and a magnetic detecting element placed in the core gap.
As the magnetic detecting element of the current sensor, a magneto-resistance effect element (GMR element, TMR element) including a laminate structure having a fixed magnetic layer with a fixed magnetizing direction, a nonmagnetic layer, and a free magnetic layer with a magnetizing direction varied with respect to an external magnetic field, or the like is used. Such a current sensor includes a full-bridge circuit constituted by a magneto-resistance effect element and a fixed resistance element (Japanese Unexamined Patent Application Publication No. 2007-248054).
As the current sensor including a magnetic detecting bridge circuit (magnetic field detecting bridge circuit) constituted by the magneto-resistance effect element and the fixed resistance element, for example, there is a magnetic balance current sensor shown in FIGS. 16 and 17. The magnetic balance current sensor measures a current to be measured based on a current flowing in a feedback coil when the feedback coil is energized by a voltage difference obtained by a magnetic detecting bridge circuit 2, and then an induced magnetic field generated by the current I to be measured which energizes a conductor 1, and a cancelling magnetic field generated by the feedback coil are in an equilibrium state in which they are cancelled.
The magnetic detecting bridge circuit 2 of the current sensor shown in FIG. 16 includes one magneto-resistance effect element 201 and three fixed resistance elements 202a to 202c. In the magnetic detecting bridge circuit 2, a resistance value of the magneto-resistance effect element 201 on a zero magnetic field is identical to a resistance value of the fixed resistance elements 202a to 202c (Rcom). In addition, an output between the fixed resistance elements 202b and 202c is set to Out1, and an output between the magneto-resistance effect element 201 and the fixed resistance element 202a is set to Out2. In addition, a resistance value of the fixed resistance element 202b is set to R1, a resistance value of the fixed resistance element 202a is set to R2, a resistance value of the fixed resistance element 202c is set to R3, and a resistance value of the magneto-resistance effect element 201 is set to R4.
When a resistance change amount of the magneto-resistance effect element 201 according to the induced magnetic field generated from the current I to be measured is ΔR, a midpoint potential difference (Out1 and Out2) of the bridge is obtained as follows:Resistance between Vdd and Gnd1=R1+R3=2×Rcom Resistance between Vdd and Gnd2=R2+(R4−ΔR)=2×Rcom−ΔR Potential of Out1=(Rcom)/(2×Rcom)×Vdd Potential of Out2=(Rcom−ΔR)/(2×Rcom−ΔR)×Vdd Potential difference between Out1 and Out2=ΔR/{2×(2×Rcom−ΔR)}×Vdd 
The magnetic detecting bridge circuit 2 of the current sensor shown in FIG. 17 includes two magneto-resistance effect elements 201a and 201b, and two fixed resistance elements 202a and 202b. In the magnetic detecting bridge circuit 2, resistance values of the magneto-resistance effect elements 201a and 201b are equal to resistance values of the fixed resistance elements 202a and 202b (Rcom). In addition, resistance change rates of the magneto-resistance effect elements 201a and 201b are equal to each other. Moreover, the output between the magneto-resistance effect element 201b and the fixed resistance element 202b is set to Out1, and the output between the magneto-resistance effect element 201a and the fixed resistance element 202a is set to Out2. Further, a resistance value of the magneto-resistance effect element 201b is set to R1, a resistance value of the fixed resistance element 202a is set to R2, a resistance value of the fixed resistance element 202b is set to R3, and a resistance value of the magneto-resistance effect element 201a is set to R4.
When a resistance change amount of the magneto-resistance effect elements 201a and 201b by the induced magnetic field generated from the current I to be measured is ΔR, a midpoint potential difference (Out1 and Out2) of the bridge is obtained as follows:Resistance between Vdd and Gnd1=(R1−ΔR)+R3=2×Rcom−ΔR Resistance between Vdd and Gnd2=R2+(R4−ΔR)=2×Rcom−ΔR Potential of Out1=(Rcom)/(2×Rcom−ΔR)×Vdd Potential of Out2=(Rcom−ΔR)/(2×Rcom−ΔR)×Vdd Potential difference between Out1 and Out2=ΔR/(2×Rcom−ΔR)×Vdd 
However, in the configuration of the magnetic detecting bridge circuit shown in FIGS. 16 and 17, a term ΔR is contained in a denominator in the equation of the midpoint potential difference of the bridge. For this reason, there is a problem that the output of the midpoint potential difference is not varied completely in proportion to the induced magnetic field which is generated by the current I to be measured, and thus the measurement accuracy is deteriorated.